Government forces seized large parts of the key town of Saraqib on Thursday as they pushed an offensive in the last rebel enclave in north-west Syria.
Weeks of intensive aerial bombardment and a bruising ground offensive have emptied entire towns in the Idlib region and sent hundreds of thousands fleeing their homes.
The battle for Saraqib, which sits at the junction of two major highways coveted by the regime, came as Turkey sent reinforcements north of the town overnight, a day after Ankara warned pro-Damascus forces to back off.
After breaking into Saraqib on Wednesday, Syrian regime forces on Thursday seized large sectors of the town after a day of heavy battles against rebels and extremists, the Syrian Observatory for Human Rights said.
The war monitor in Britain said the forces pushed back a "counter-offensive" by opposition fighters in Saraqib, a town of 110,000 residents now almost deserted after months of bombardment.
Syria's state news agency Sana confirmed the report, saying government troops were carrying out "mopping up" operations and "dismantling mines and explosives" planted there by the rebels and extremists.
The UN Security Council held an emergency session in New York after clashes this week between the Syrian and Turkish armed forces.
The UK’s representative to the council condemned the offensive, calling it a “vicious campaign of attrition”.
Karen Pierce said Idlib was “the refuge of last resort for all those people who had to flee the attacks on Aleppo and elsewhere”.
Tension between Ankara and Damascus mounted on Monday after Turkish and regime forces exchanged deadly fire in a rare escalation that killed more than 20 people on the two sides.
Video: Turkey returns fire after Syria bombing
The increase in fighting in the Idlib region, home to about 3 million people, has killed more than 300 civilians since mid-December, including 17 on Thursday, the Observatory said.
Ten of those killed on Thursday died in an air strike on the eastern outskirts of Idlib city, the monitor said.
The UN and aid groups have called for an immediate end to the violence, which has pushed around 586,000 people from their homes in two months.
Turkey, which has troops in north-west Syria where it backs the rebels, has repeatedly called on Damascus and its allies to stop the offensive against Idlib.
On Wednesday Turkish President Recep Tayyip Erdogan again demanded an end to the Syrian push into Idlib.
"If the regime does not pull back, Turkey will be obliged to take matters into its own hands," Mr Erdogan told a meeting of ruling party politicians in Ankara, giving Syria until the end of the month to comply.
Early on Thursday, Turkey said its troops had arrived in the area of Taftanaz, about 16km north of Saraqib.
The Syrian army in a statement accused the Turks of obstructing its advance and "impeding it from completely stamping out the organised terrorism besieging civilians in Idlib province".
The Observatory also reported clashes on Thursday to the north, west and south of Saraqib, backed by Turkish artillery fire. Ankara did not confirm its involvement.
Under a 2018 deal with Moscow, Turkey has 12 observation posts in Idlib aimed at preventing a regime offensive.
The Damascus regime controls more than 70 per cent of Syrian territory and has repeatedly vowed to reclaim the entire country, including Idlib. It has taken a string of villages and towns in recent weeks.
The region, and nearby areas of Aleppo and Latakia provinces, are dominated by the Hayat Tahrir Al Sham extremist group, led by members of the country's former Al Qaeda franchise.
Pre-dawn air strikes by Israel's military on Thursday killed 23 Syrian and foreign fighters.
Three Iranians and seven Tehran-backed foreign fighters were killed near Kisweh south of the capital, the Observatory said.
Eight Syrian air defence personnel died in Mezzeh and Jisr Baghdad, west of the capital, it said.
Five Syrian members of a pro-Iran group were killed in the Ezra area in the southern province of Deraa.
A Syrian army source quoted by Sana said air defences responded to two waves of Israeli strikes after midnight.
An Israeli army spokesman declined to comment on the strikes, but Israel has carried out repeated strikes in Syria since the civil war erupted, mainly against government forces and their Iranian and Hezbollah allies.
Syria's war has killed more than 380,000 people and displaced millions since it started in 2011 with the brutal repression of protests against President Bashar Al Assad.
